Introduction
Pollution of water by intensive farming continues to be cause for
concern for the physicochemical and ecological health of freshwaters
(Mateo-Sagasta et al., 2017). Contaminants moving from agricultural land
into freshwater systems include fine-grained sediment (Pulley and
Collins, 2019), deliberately applied compounds, e.g., fertilizers such
as urea (Gilbert et al., 2005), ammonium nitrate (Burt et al., 2011),
phosphorus (Haygarth et al., 2005) and other products such as pesticides
(Syafrudin et al., 2021). In turn, these emissions not only degrade
water quality but also impact detrimentally on freshwater ecology across
all trophic levels (Collins et al., 2011; Kemp et al., 2011; Jones et
al., 2012a,b; Jones et al., 2014).
Buffer strips have been utilised as a means of reducing the movement of
pollutants from agricultural land into watercourses for many years
(Barling and Moore, 1994; Hickey and Doran, 2004). The form of the
vegetation may take that of a grass verge at the edge of the field where
no targeted planting of chosen species is undertaken and natural
colonisation is allowed to determine the dominant form of vegetation.
Alternatively, targeted planting of specific grasses and woody plants
has been utilised to vegetate buffer strips, with consequent effects on
landscape aesthetics, biodiversity and interactions with the local
watercourse and its ecology (Cole et al., 2020). Choices of the type of
plants that can be deliberately planted within a buffer strip range from
herbaceous grasses and forbs to small woody shrubs with multiple stems
to taller woody tree species. The physiognomy of the plants may affect
the runoff, the movement of pollutants including fine-grained sediment
or both (Roberts et al., 2012). The interaction-potential the buffer
strip has for removing pollutants from the runoff leaving the field
upslope may thus change depending on the form of planting used to
vegetate the buffer strip. Here, the form of planting chosen may affect
the ability of the buffer strip to remove a priority pollutant within a
local area, and as a result, some degree of potential exists to optimise
buffer vegetation to ameliorate specific local concerns over particular
pollutants or, alternatively, to address multiple issues (Stutter et
al., 2012).
Water pollution and flooding events associated with the movement of
agricultural run-off have been reduced due to the interaction of water
and vegetation within buffer strips. However, the ability of a buffer
strip to provide such services continuously may be reduced or lost over
time if the buffer strip becomes saturated with fine-grained sediment or
nutrients (Valkama et al., 2019). To alleviate the potential for
saturation of nutrients, planned removal of buffer strip vegetation can
be implemented. For grass buffer strips, mowing and/or grazing can
reduce the standing crop within the strip and cause compaction by
trafficking or trampling. Access to strips may negate the possibility of
using machinery in some circumstances (e.g., steeply sloped land), and
refusal by grazing animals to consume standing vegetation may affect the
amount of vegetation removed. The age of a grass dominated buffer strip
may need to be considered if grazing animals are the only option
available to reduce the standing crop. Woody plants can be harvested for
their timber within a planned management system, and act as a means of
both removing nutrients from the strip as well as reinvigorating plant
growth rates, and thus facilitating the further removal of nutrients
entering the buffer strip.
In England, implementation of water pollution interventions on farms,
including buffer strips, is driven by a combination of regulation,
incentivization in the form of agri-environment schemes and the delivery
of on-farm advice for win-wins. Here, improved uptake rates by farms can
be encouraged by robust scientific evidence on the efficacy of buffer
strips for controlling runoff and pollution losses. Existing work
examining the efficacy of buffer strips for environmental good has
focussed on both external and internal factors (Eck, 2000). The former
encompass the phase (i.e., particulate, dissolved) and delivery pathway
(i.e., surface, subsurface) of the incoming pollution, whereas the
latter include buffer width and vegetation cover. Advice delivery has
tended to focus on buffer width in the case of internal controls since
this is the easiest component of management to influence via farm
management and existing evidence on varying efficacy for runoff and
water pollution reduction, including width, can be readily extracted
from a number of comprehensive reviews (e.g., Barling & Moore, 1994;
Hickey and Doran, 2004; Dorioz et al., 2006; Kay et al., 2009; Collins
et al., 2009). Beyond buffer strip width, the existing evidence on the
effects of different vegetation cover remains less easy to generalise.
Some work suggests that for the same buffer strip width, different
vegetation cover impacts efficacy for pollution control by at most 20%
(Dorioz et al., 2006). Other studies report very limited or no effect of
vegetation cover (e.g., Schmitt et al., 1999; Uusi-Kamppa et al., 2000).
In other cases, the results of investigations comparing herbaceous and
woody vegetation in buffer strips report both a lack of (Syversen, 1995;
Daniels and Gilliam, 1996) and detectable (Cooper et al., 1986; Parsons
et al., 1994) differences in pollution reduction, with the latter
suggesting better performance by herbaceous cover.
Given the above context, the new study detailed herein was undertaken to
assess the impact of three different vegetated buffer strips on runoff
and sediment loss to contribute to the evidence base. The research
project was planned to provide replicated evidence on buffer strip
efficacy and to engage multiple stakeholders with this evidence given
the ongoing inclusion of buffer strips in agricultural policy in the UK
and beyond. This paper reports the preliminary results for the efficacy
for reductions in sediment loss using our new dataset.